In recent years, with the popularization of smart phones, the number of servers deployed in a data center is vastly increased. For steady operation of the servers, a computer room air conditioning (CRAC) system is used to keep the servers at a low temperature. As shown in FIG. 1, a server rack has a rack body 9 defining a storage space 90, and a plurality of servers 91 are disposed in the storage space 90 and held by the rack body 9. A passageway 93 is defined between two adjacent ones of the servers 91. Cooling air is drawn into the storage space 90 by a fan (not shown) and is directed to pass through the passageway 93 along an arrow (P) to cool the servers 91. However, the cooling of the servers 91 entails the consumption of significant power. Therefore, there is a need to improve the efficiency of the cooling of the servers 91.